Privacy rights in the digital age face a crucial test Wednesday when the Supreme Court hears a case over police use of a person's location data from cellphone towers.

Civil liberties advocates from both the right and the left have called for the US high court to agree that privacy rights guaranteed by the US Constitution's Fourth Amendment extend to electronic data held by third parties -- phone companies in this case -- but by implication any data held by social media companies or stored in the digital cloud.

But law enforcement authorities, faced with the challenges of phones used under false identities and with unbreakable encryption, say access to such data is crucial to fighting crime.

"This case will likely decide whether the 4th Amendment is to retain its vitality in the digital age, as it becomes virtually impossible for any of us to go about our daily lives without leaving trails of digital data behind us," said Nathan Wessler, a staff attorney with the American Civil Liberties Union.

The specific case under review, Carpenter vs US, involves the arrest of a man named Timothy Carpenter in 2011. To investigate a spate of armed holdups, police studied cell tower data going back four months and it eventually linked Carpenter to the robbery locations.

The police had obtained some 12,898 location points on Carpenter from two phone companies. Carpenter challenged his conviction, saying it hinged on private information that should not have been provided without a court-issued warrant. The data, for example, showed when he slept at home and elsewhere, and when he was in a church.

- Constitutional test case -

The Fourth Amendment guarantees the privacy of citizens from "unreasonable searches and seizures", and says police must obtain warrants based on "probable cause" if they want to search a suspect's "persons, houses, papers, and effects."

Parties on both sides of the case agree that the law did not anticipate an era when everyone relies on a cellphone and that technology providers can amass data on a person from their phone use.

"This is a challenging case.... On one hand, the law must keep up as technology changes to maintain privacy protections. On the other hand, the blunt instrument of the Fourth Amendment shouldn't be forced beyond its proper role," wrote Orin Kerr, a law professor at George Washington University Law School who backs the government in this case.

A broad range of private and industry groups submitted arguments to the court detailing why the nine justices should extend privacy protections to the kind of data the case focuses on.

They note that a whole range of devices used by people today, from cellphones to home appliances to watches and baby monitors, automatically deliver data to technology and communications companies which store it away.

- 'Unreasonable' search? -

The government and law enforcement officials argue this data should be accessible to police because it is not addressed by the Fourth Amendment, and is not truly private, since it is in the hands of third parties. Getting cell tower data is no different than collecting information from human witnesses in the public space, they say.

They also say that asking for it as part of an investigation is not an "unreasonable" search banned by the Constitution.

But civil libertarian groups reject that argument, saying the data gives far more information on a person than what a single witness could say.

"Even in limited quantities, cell phone location information can be a telltale sign of private social, political, and religious activities protected by the Constitution," said the Brennan Center For Justice.

All agree, though, that what the court eventually decides, which could take months, will set a standard on digital privacy for years if not decades.

"I don't think it's an exaggeration to say that is the most important Fourth Amendment case that we have seen in a generation," said Wessler.

"It really promises to decide whether the mere use of modern technologies is going to make our lives into an open book for the government, without the protection of a warrant."